The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A variety of intrusive and non-intrusive pressure sensing means are known for sensing pressure within an internal combustion engine cylinder when the engine is motoring and when the engine is firing.
Combustion timing or phasing is useful to diagnose issues in the combustion process. Examining mass fraction burn is one known method to examine combustion phasing for a cylinder. Mass fraction burn is an estimate of how much of the charge within the combustion chamber of a cylinder has been combusted at a given crank angle. For a normal combustion process operated under a particular set of parameters, mass fraction burn is predictable to within a small range. Mass fraction burn values indicating deviation from this small range indicate that conditions within the combustion chamber are outside of the expected parameters. Mass fraction burn is estimated in a number of ways.
One known method to estimate mass fraction burn includes examining pressure data from within the combustion chamber, including analyzing the pressure rise within the chamber attributable to combustion. Various methods exist to quantify pressure rise in a cylinder attributable to combustion. Pressure ratio management (PRM) is a method based upon the Rassweiler approach, which states that mass fraction burn may be approximated by the fractional pressure rise due to combustion. Combustion of a known charge at a known time under known conditions tends to produce a consistently predictable pressure rise within the cylinder. PRM derives a pressure ratio from the ratio of a measured cylinder pressure under combustion at a given crank angle to a calculated motored pressure, estimating a pressure value if no combustion took place in the cylinder, at a given crank angle. Any rise in pressure above the motored pressure is attributable to energy introduced by combustion. This method therefore describes the combustion process within a cylinder, including combustion phasing information, and normalizing the pressure ratio value at a given crank angle to a completed combustion cycle yields a fractional pressure ratio estimating the mass fraction burn.
The Rassweiler approach may also be used directly to determine mass fraction burn. This approach quantifies the pressure rise in a combustion chamber attributable to combustion and sums the rise over a range of crank angles. This accumulated sum of pressure rise attributable to combustion over a present combustion cycle can be compared to an expected total to yield the fractional portion of the charge that has been combusted or the mass fraction burn.
Another method to estimate mass fraction burn is through classical heat release models based on the First Law of Thermodynamics. Known equations equate a rate of heat release in combustion to measured cylinder readings. Integration of this heat release rate yields a net energy release for a given crank angle. This net energy release may be compared to an expected energy release to yield the fractional portion of the charge that has been combusted or the mass fraction burn.
Compression-ignition engines and other engine control schemes operate over broad engine conditions. Effective control, including fuel control, fuel tailoring, charge ignition timing control, exhaust gas recirculation (EGR) control, is necessary to meet operator demands for performance and fuel economy and comply with emissions requirements. Furthermore, there is much variability, including that related to: components, e.g., fuel injectors; systems, e.g., fuel line and pressures; operating conditions, e.g., ambient pressures and temperatures; and, fuels, e.g., cetane number and alcohol content. The variability in combustion affects heat release and work output from individual cylinders, resulting in non-optimal performance of the engine. Any change in the engine performance is apparent in cylinder pressure ratios. A measure of combustion variability would be valuable to diagnose instability in the combustion process and providing information useful to reduce periods of inefficient or high emission operation.